Paperboard can be coated with one or more coating layers in order to improve its printability. The more coating layers are applied, the higher the attained paper quality, simultaneously enabling the use of coating agents having different properties. However, the use of two coating layers entails a significant increase in production costs because the coating is usually carried out in separate steps, and the applied coating layer has been dried prior to the application of the next layer.
The coating can be performed either directly onto the board surface by means of, e.g., a blade or a rod coater, or by means of a film transfer coater whereby film which has been apportioned onto a film roll is transferred onto the paper surface in a roll nip. It is typical of the blade and rod coating methods that the doctor blade fills the roughness volume of the coating underlayer and evens out the surface whereby the thickness of the coat varies in accordance with the roughness volume variations of the coating underlayer. A smooth coat results, having an uneven brightness coverage, and the uniform absorption properties of the coat are difficult to control.
In film transfer coating, a coat of an essentially more uniform thickness is obtained rendering it easy to control the absorption properties, but sufficient smoothness properties pose a problem, particularly in the case of thicker papers and boards. The coating also provides quite an even coverage, whereby, for example, the brightness of a coating underlayer of low brightness can be significantly improved by means of this method. Furthermore, as no doctor blade trailing along the coating underlayer is involved in film transfer coating, the method offers excellent runnability with respect to coating breaks.
When paperboard is manufactured furnished with two or several coating layers, the investment costs involved are significant, because the coated web must be dried prior to the application of the next layer. This results in great length of the coating line, wherefore it requires a lot of room in the factory hall. The drying also requires a significant amount of thermal energy. Thus, it would be of advantage to apply the second coating layer onto a wet or damp first layer without any intermediate drying step. This, however, leads to problems in that the application and evening out of the second coating layer scrapes most of the first wet coating layer off the board surface and a thin final coat results, whereby at least some of the advantages of the duplex coating are lost.